This invention relates to surface wave devices. The term xe2x80x9csurface wavexe2x80x9d is used herein to embrace surface acoustic waves (SAWs), including leaky SAWs, surface skimming bulk waves, and similar acoustic waves, and is abbreviated to SAW below.
It is known to provide a SAW device with tapered interdigital transducers in order to provide a changing SAW wavelength, and hence a changing center frequency for SAW propagation, across the aperture of the SAW device, thereby to provide the SAW device with a relatively broad fractional bandwidth. The taper may be linear, hyperbolic, or in accordance with some other function, and tapers can be applied to IDTs of different types, for example to bidirectional IDTs and to SPUDTs (single phase unidirectional transducers) with and without known weighting techniques such as withdrawal weighting, finger position or width weighting, etc. SAW device filters using tapered IDTs can have advantages of low loss, a flat pass band, and good out-of-band rejection.
Tapered IDTs and SAW devices using them have also been referred to as having slanted finger geometries, but the term xe2x80x9cslantedxe2x80x9d is not used further herein to avoid potential confusion with other and different types of SAW device, such as slanted array compressors (SACs).
The gradually changing finger pitch across the aperture of a known tapered IDT requires a substantially increased resolution of the processes used for producing the IDT. For SAW devices intended for operation at high frequencies, such an increased resolution becomes increasingly difficult or impossible to provide.
For example, a SAW device for use as an RF (radio frequency) filter at a frequency of the order of 2 GHz may have a finger width of about 0.3 xcexcm. To provide such an IDT with a tapered finger geometry with for example a fractional bandwidth of 1%, the finger width would vary from about 0.3 xcexcm across the aperture of the IDT by xc2x10.0015 xcexcm. The resolution limits of known SAW device processes can not practically meet the requirements for such a tapered finger geometry.
It would be desirable to be able to obtain the advantages of tapered IDTs in high frequency SAW devices, but this has not been possible in view of the resolution limits and requirements discussed above.
According to one aspect of this invention, a method of providing a tapered IDT function in an IDT of a SAW device having fingers substantially parallel to one another comprises applying a static electric field to a piezoelectric substrate of the SAW device to generate in said substrate a strain which varies across the aperture of the IDT.
Thus in a SAW device which operates in accordance with this method, the IDT fingers are parallel to one another, perpendicular to the direction of SAW propagation, as is conventional for a non-tapered IDT, so that an increased resolution is not required for producing the SAW device.
Instead, a static electric field is created, conveniently by applying a dc voltage to one or more additional conductors provided for this purpose on the surface of the piezoelectric substrate, in such a manner that, due to the piezoelectric effect of the substrate, there is a strain in the region of the IDT, this strain varying across the aperture of the IDT so that the SAW propagation velocity is different at different points across the aperture of the IDT.
Thus instead of geometrically changing the finger pitch, and hence center frequency SAW wavelength, across the aperture of an IDT as is the case for a known tapered IDT, the finger pitch is maintained geometrically constant and the SAW propagation velocity is changed across the aperture as a result of the strain due to the electric field. As the SAW device frequency is equal to the SAW propagation velocity divided by the SAW wavelength, a similar result can be obtained in that the SAW device center frequency is varied across the aperture of the IDT.
The substrate in the region of the IDT can be directly subjected to the electric field, directly creating a strain in this area, with the variation in the electric field being conveniently provided by a dc voltage applied to a tapered or angled conductor. Unlike the tapered finger geometry of a tapered IDT, the taper or angle of such a conductor does not require an increased resolution. Different electric field strengths could alternatively be created in different regions across the aperture of the IDT by applying different voltages to respective conductors.
Alternatively, the electric field can be created in an area of the substrate surface adjacent to the IDT, thereby directly producing a strain in this area, this strain indirectly resulting in a strain in the region of the IDT and varying across its aperture. The small size of high frequency SAW devices facilitates this result of an indirectly produced strain. This has the advantage that conductors for applying a dc voltage to create the electric field are separated from the conductors of the IDT. As described further below, the indirect strain can be produced with an electric field created either at one end of the IDT or laterally at one or both of the sides of the IDT.
It will be appreciated that-these techniques can be combined, among themselves and/or with other techniques (e.g. a tapered finger geometry) known in the art, to produce further results. In addition, it will be appreciated that a dc voltage used for creating the static electric field may be varied to provide tuning of the frequency characteristics of the SAW device.
Another aspect of the invention provides a method of varying a center frequency-of an IDT (interdigital transducer) of a SAW (surface wave) device across an aperture of the IDT, comprising applying a static electric field to a piezoelectric substrate of the SAW device to generate in said substrate a strain which varies across the aperture of the IDT.
In this method, the electric field can be applied to the substrate in the region of the IDT and in a direction substantially perpendicular to fingers of the IDT by applying a voltage between conductors on the substrate to produce an electric field strength varying across the aperture of the IDT; the conductors can comprise at least one angled or tapered conductor.
Alternatively, the electric field can be applied to the substrate in an area adjacent to the IDT to produce indirectly a strain varying across the aperture of the IDT. For example, the electric field can be created by applying a voltage between conductors adjacent to an end of the IDT, the electric field having a direction substantially perpendicular to fingers of the IDT and varying across the aperture of the IDT, or by applying a voltage between conductors adjacent to a side of the IDT, the electric field having a direction substantially-parallel to fingers of the IDT. Further, two electric fields-can be created by applying a voltage between conductors on the substrate adjacent to each side of the IDT, the two electric fields having directions opposite to one another and substantially parallel to fingers of the IDT to produce indirectly a strain varying across the aperture of the IDT.
The invention also provides a SAW (surface wave) device comprising: a piezoelectric substrate; at least one IDT (interdigital transducer) on a surface of the substrate, the IDT having interdigital fingers substantially parallel to one another for propagation of a SAW in a direction substantially perpendicular to the fingers within an aperture of the IDT; and
at least one conductor on the surface of the substrate for receiving a dc voltage to create a static electric field to produce in said substrate a strain which varies across the aperture of the IDT thereby to vary a center frequency of the IDT across the aperture.
Said at least one conductor can comprise conductors adjacent to ends of the IDT for establishing said electric field in the region of the IDT with an electric field strength which varies across the aperture of the IDT, or conductors in an area adjacent to and end of the IDT for establishing said electric field in said area with a direction substantially perpendicular to the fingers and varying across the aperture of the IDT, whereby a strain varying across the aperture of the IDT is produced indirectly in a region of the IDT.
Alternatively, said at least one conductor can comprise conductors adjacent to each side of the IDT for establishing two electric fields having directions opposite to one another and substantially parallel to fingers of the IDT, whereby a strain varying across the aperture of the IDT is produced indirectly in a region of the IDT.